It is well known in the papermaking industries to use vacuum or suction pipe systems in dewatering. Such systems often utilize suction pipes coupled to elongated suction slots over which a felt passes causing the dewatering thereof.
In systems of this type, a variety of devices exist to create the vacuum necessary for dewatering. For example, liquid ring pumps, positive displacement pumps and centrifugal exhausters or blowers. While many circumstances and operating parameters dictate what type device in this regard is best suited for a particular application, a common desire in which ever type vacuum pump is selected is that it be efficiently incorporated and utilized in the system.
In this regard, generally the vacuum pumps are sized for maximum demand vacuum conditions in the suction pipe when the felt is new. The vacuum pump will normally run at its maximum speed with a new felt. As a felt fabric fills voids during its life, it becomes less permeable requiring a higher vacuum level for dewatering. However, with the decreased felt permeability and since the vacuum pump is a constant volume unit, the vacuum level will automatically increase.
Heretofore, many systems have been devised to take advantage of increasing vacuum conditions and to effect cost and energy saving as a result thereof. See i.e., U.S. Pat. Nos. 4,308,077, issued Dec. 29, 1981; 4,329,201, issued May 11, 1982; and 4,398,996, issued June 19, 1981. For example, in the variable vacuum liquid ring pump having constant flow, as set forth in U.S. Pat. No. 4,398,996, a variable speed drive motor is provided and is responsive to an increase in the vacuum level in the suction pipe. The motor is activated to slow down the vacuum pump as the felt permeability decreases, thereby retaining the desired level in the suction pipe. The lower pump speed results in lower drive horsepower and accordingly a savings in power, while retaining the desired vacuum level in the suction pipe.
While such an arrangement has proven satisfactory in certain applications, it is desired to provide for yet further efficiency and energy savings in a dewatering system, particularly one that utilizes a centrifugal exhauster or blower as compared to a positive displacement unit.